fuzzing.yml: Start producing code coverage report

.github/workflows/fuzzing.yml

@@ -87,7 +87,7 @@ jobs:
             # Tune compilation of fuzzers to use Clang with ASan and UBSan
             -DCMAKE_C_COMPILER=clang
             -DCMAKE_CXX_COMPILER=clang++
-            -DCMAKE_{C,CXX}_FLAGS='-Wall -Wextra -pedantic -O1 -g -fsanitize=address,undefined -fno-sanitize-recover=all -fno-omit-frame-pointer -fno-common'
+            -DCMAKE_{C,CXX}_FLAGS='-Wall -Wextra -pedantic -O1 -g -fsanitize=address,undefined -fno-sanitize-recover=all -fno-omit-frame-pointer -fno-common -fprofile-instr-generate -fcoverage-mapping'
             -DCMAKE_{EXE,MODULE,SHARED}_LINKER_FLAGS='-g -fsanitize=address,undefined'
             -DEXPAT_WARNINGS_AS_ERRORS=ON
         )
@@ -119,6 +119,9 @@ jobs:
         set -x -o pipefail
         cd expat/build/
 
+        mkdir coverage/
+        export LLVM_PROFILE_FILE=coverage/expat-%p.profraw
+
         #           vvvvv
         find corpus_UTF-8/ -type f | sort | xargs \
             fuzz/xml_parse_fuzzer_UTF-8 "${fuzz_args[@]}"
@@ -134,3 +137,25 @@ jobs:
         name: expat_fuzzing_logs_${{ github.sha }}
         path: expat/build/fuzz-*.log
         if-no-files-found: error
+
+    - name: Render coverage report
+      run: |
+        fuzzer=xml_parse_fuzzer_UTF-8  # could be any of them
+
+        set -x -o pipefail
+        cd expat/build/
+
+        # Merged and convert to a single indexed profile data file
+        llvm-profdata merge -sparse -o coverage/expat.profdata coverage/expat-*.profraw
+
+        # Render report
+        llvm-cov show fuzz/${fuzzer} -instr-profile=coverage/expat.profdata -show-branches=count -format=html -output-dir=coverage/html/
+        llvm-cov report fuzz/${fuzzer} -instr-profile=coverage/expat.profdata -show-functions -sources ../lib/ | tee coverage/report_functions.txt
+        llvm-cov report fuzz/${fuzzer} -instr-profile=coverage/expat.profdata                 -sources ../lib/ | tee coverage/report_files.txt
+
+    - name: Store coverage report
+      uses: actions/upload-artifact@65c4c4a1ddee5b72f698fdd19549f0f0fb45cf08  # v4.6.0
+      with:
+        name: expat_fuzzing_coverage_${{ github.sha }}
+        path: expat/build/coverage/
+        if-no-files-found: error